Evaluation of gill nets, fyke nets, and mark-recapture methods to estimate the number of Hirudinea and Crustacea on fish.

Twenty species of fishes (n = 20,759) were collected from Dauphin Lake, Manitoba, Canada, to determine the types and numbers of ectoparasites they harbored. Counts of ectoparasites on fishes collected with different gear were compared to evaluate different methods of collection and to estimate rates of recruitment of ectoparasites by fishes. Ectoparasites were found on 11 species of fishes and the majority of these were parasitic leeches (Myzobdella moorei, Cystobranchus verilli, and Placobdella montifera) and parasitic Crustacea (Argulus appendiculosus and Lernaea cyprinacea). Some fishes also were infested by neascus-type metacercariae (blackspot) or had tumors (lymphocystis). The prevalence of ectoparasites was correlated with the abundance, feeding habits, and spatial distribution of fish species. Argulus appendiculosus and blackspot were more prevalent on benthic fishes, whereas M. moorei and tumors were more prevalent on limnetic fishes. Mark-recapture records showed that fishes occupying shallow (less than or equal to 1.5 m) water had a higher prevalence of infestation and 28 of 29 infected fishes caught by gill nets were captured in shallow water. Placobdella montifera was the only ectoparasite found on fishes from deep (1.5-3.5 m) water and the only species that was acquired by fishes previously released with no ectoparasite (2 of 239 fishes). The littoral zone (less than or equal to 1.5 m) comprises only 14% of the surface area and 3% of the volume of Dauphin Lake, yet 72% of all gill-netted fishes harboring ectoparasites were collected there. Intensities of ectoparasites estimated from gill net and pound net samples were similar, but prevalence of ectoparasites estimated from samples obtained with gill nets was lower.